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

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 chrysotile samples, crocidolite, tremolite asbestos, amosite, anthophyllite asbestos and asbestiform erionite. Powder X-ray diffraction and scanning electron microscopy 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.

Amphiboles are double-chain silicates which may display a fibrous habit being structurally elongated in one preferred crystal direction. Finally, erionite is a common fibrous/acicular zeolite with an hexagonal, cage-like structure composed of a framework of linked tetrahedral [5,6].
Despite their outstanding technological properties (e. g. low thermal conductivity, high mechanical strength, workability, among the others) which prompt their widespread industrial applications, asbestos minerals are considered hazardous. In general, all asbestos fibres if inhaled are thought to induce malignant mesothelioma, lung cancer (in combination with other factors), and other lung diseases [7,8]. According to the existing regulations, amphibole asbestos fibres are banned worldwide whereas chrysotile is banned in only 28% of the countries worldwide. In the other countries, safe use of chrysotile is admitted. Asbestos erionite unfortunately is not regulated but listed by International Agency for Research of Cancer (IARC) as substance carcinogenic to humans.
As a matter of fact, in vivo studies unequivocally proved that asbestiform erionite is more tumorigenic than chrysotile and crocidolite asbestos [9].
Since the advent of industrial age, asbestos fibres have been extensively used in an endless number of industrial applications and especially to manufacture various types of artefacts (asbestos cement, disc brake pads, pipes, reinforcing agents, fire retardants etc). In those countries where all asbestos minerals are banned, and remediation policies are fostered, many attempts were made to detoxify asbestos minerals by using different techniques [10][11][12]. In this regard, many projects and patents have dealt with the possible disposal and re-use of asbestos-containing materials (ACM) via the crystal-chemical transformation induced by thermal treatment [13][14][15][16][17][18]. However, it must be considered that the thermal transformations sequence of asbestos-containing materials (ACM), composed of a variety of different crystalline and amorphous phases, is totally different with respect to the transformations sequence of pure asbestos minerals.  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 Concerning pure chrysotile, it has been demonstrated that its structure collapse at around 650 °C with early recrystallization at about 800 °C into anhydrous silicates such as forsterite and amorphous silica [19]. With respect to chrysotile, papers dealing with the thermal decomposition of pure amphibole asbestos and erionite are rare. As a matter of fact, only few studies were devoted to the thermal decomposition of pure amphibole asbestos up to 1100 °C and rare data report on the new phases appeared after thermal treatment. Thermal decomposition of fibrous amphibole minerals have been generally limited to asbestos minerals of commercial value or health concern such as crocidolite [20][21][22][23][24][25] and amosite [23, 26,27]. Moreover, in some papers regarding the thermal analysis of amphiboles asbestos, a full picture of their thermal behaviour is not given [28].
Regarding erionite, the thermal behaviour of a sample from Jersey Nevada (USA) is described in only one paper [29] which showed that the main endothermic event has occurred at about 140 °C but did not report the TG curve neither the temperature of the structural collapse.
In this scenario, the aim of this study was to systematically investigate and compare the thermal behaviour (TG/DSC) as well as the phase transformations of the most relevant mineral fibres during heating up to 1000 or 1100 °C. The study was performed on eight selected fibrous minerals including chrysotile, crocidolite (asbestiform riebeckite), tremolite asbestos, anthophyllite asbestos, amosite (grunerite asbestos) and fibrous erionite object of many important biomedical studies [30,31]. Four fibrous species (chrysotile, crocidolite, amosite, and anthophyllite asbestos) were distributed by the International Union Against Cancer (UICC).

Materials and methods
TG and DSC were performed in an alumina crucible under a constant nitrogen flow of 30 cm 3 min -1 with a Netzsch STA 449 C Jupiter in a 25 -1000 and 25 -1100 °C temperature range, with a heating rate of 10 °C/min. Instrumental precision was checked by six repeated collections on a kaolinite reference sample revealing good reproducibility (instrumental theoretical T precision of ± 1.2 °C) and theoretical weight sensitivity of 0.10 µg, DSC detection limit < 1 µW. Samples were powdered by dry-grinding in an agate mortar; about 40 mg of each sample were used in all collections. Owing to the remarkable length of the chrysotile fibres from Val Malenco, before grinding they were cut with scissors.
A qualitative phase analysis, both of natural and heated samples, was performed according to the powder X-ray diffraction method (PXRD) using a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA. The instrument is equipped with a copper tube and curved graphite monochromator.
Scans were recorded in the range of 3-66 °2θ, with a step interval of 0.02 °2θ and a step-counting time of 3 s/step. EVA software (DIFFRACplus EVA) was used to identify the mineral phases and experimental peaks being compared with the 2005 PDF2 reference patterns. The morphology of the samples before and after thermal analysis was investigated by scanning electron microscopy (SEM) using an Environmental Scanning Electron Microscope FEI QUANTA 200 equipped with an EDAX Genesis 4000 energy dispersive X-ray spectrometer (EDS), and a FEI Nova NanoSEM 450 equipped with an X-EDS Bruker QUATAX-200 system for the microanalysis.

Chrysotile
The thermal analysis for chrysotile UICC (Fig. 1) showed four endothermic peaks at 226, 401, 520 and 633 °C. The first peak at 226 °C may be due to the dehydroxylation of pyroaurite [33]; the peak at 401 °C to the dehydroxylation of brucite and de-oxygenation of pyroaurite [33,34]; the very weak peak at 520 °C is thought to be generated by the decarbonation of siderite [35], likely present as very minor impurity; the wide peak at 633 °C to the chrysotile dehydroxylation [22,36]. The weak endothermic event at 901 °C on DTG ( Fig. 1) curve was ascribed to talc dehydroxilation [37,38]. It is possible that the wide endothermic event at 633 °C hides minor endothermic events due to the decarbonation of calcite, dolomite, and/or dehydroxylation of clinochlore [34,36,37].
The TG curve showed a weight loss of 0.91 % below 110 °C due to adsorbed water while the main weight loss of 12.22 % was due to chrysotile dehydroxylation ( Table 2).
Our findings are in line with previous literature data on chrysotile dehydration mechanisms and high-T crystallization [39][40][41][42]. The product of the dehydroxylation of chrysotile recrystallized to forsterite [19, 43, 44] caused a sharp exothermic peak at 823 °C ( Fig. 1, Table 3). Indeed, the corresponding PXRD pattern (Fig. 2) of the chrysotile UICC after thermal treatment confirmed the presence of the forsterite.
The DSC curve of the chrysotile from Balangero exhibited one major effect (Fig. 3) at 660 °C related to the chrysotile dehydroxylation with a weight loss of 11.80 %. As already described above, the weak endothermic effect (Fig. 3, Table 3) at 402 °C is due to brucite [34] breakdown. In the DTG curve the peak at 869 °C was related to the decarbonation of dolomite [45]. The DSC weak shoulder effect at 938 °C which was clearly recorded on DDSC curve was related to the talc dehydroxilation [38]. Again, the wide endothermic event at 660 °C may hide minor endothermic events due to the I decarbonation of dolomite, calcite, and clinochlore [34,36,37]. The weak endothermic effect at 717 °C visible on the DTG curve (Fig. 3) is due to the antigorite dehydroxylation [36,46]. The effect on TG curve (-0.57 wt%) below 110 °C was ascribed to the release of humidity adsorbed at the sample surface while the total weight loss at 1000 °C was of 14.9 % ( Table 2). On the DSC curve the exothermic peak at 822 °C ( Fig. 3) was related to the crystallization of forsterite as determined by PXRD after thermal treatment at 1000 °C (Fig. 2).
The TG curve for the chrysotile from Val Malenco (Fig. 4) showed a continuous weight loss mainly due to the decomposition of chrysotile in correspondence with the major endothermic event at 652 °C (see the DSC curve in Fig. 4). Dehydroxylation of chrysotile causes a weight loss of 12.01 %.
DTG weak effect at 760 °C ( Fig. 4) is the diagnostic signal [47] of the presence of antigorite in the sample which was also detected by TEM analysis on the same sample by Cattaneo et al. [48]. Broad DTG and DSC signals in the 25-110 °C range were due to adsorbed water (weight loss of 0.54 %) while the total weight loss at 1000 °C was 13.35 % ( Table 2). A sharp exothermic peak at 820 °C indicates the crystallization of forsterite [19, 43,44] as confirmed by PXRD data (Fig. 2).
The TG curves of the three specimens of chrysotile showed a weight loss of about 12 % (Table 2) due to their decomposition [19,51]. These data match the theoretical and experimental values of mass loss observed in natural and synthetic chrysotile fibres reported in literature [19,43,47,51].
A representative set of secondary electron SEM images showing the morphology of chrysotile before and after thermal analysis at 1000 °C is reported in Fig. 6. The unheated fibres of chrysotile samples appear arranged in bundles (Fig. 6a, 6d) and curved with their typical wavy appearance  (Fig. 6b, 6e, 6h). However, at higher magnification (HM) the apparent fibres turn out to be a continuous sequence of sub-cylindrical particles with basis both sharp and perpendicular to the original fibre axis [16], approximately 100 nm in length (Fig. 6c), Sometimes, the new silicate is constituted by sub-spherical particles disposed not very tidily along the axis (Fig. 6f). The original cleavage parallel to the fibre axis is lost (Fig. 6c). Therefore, the eventual fracture of the transformed pseudo-morphic fibres occurs at the particle boundaries and not along the fibre axis.
Moreover, as observed in Fig 6i, the typical smooth surface of chrysotile fibres is completely lost becoming very rough.

Crocidolite
The DTG curve of crocidolite (Fig. 7) shows a major endothermic event at 648 °C related to iron oxidation accompanied by dehydrogenation and/or dehydroxylation. The structure does not show collapse which occurs at higher temperature [24] with a corresponding weight loss of 2.18 % in the   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 range 110-680 °C of the TG curve. No weight loss of crocidolite was observed above 700 °C in agreement with Fujishige et al. [54]. As it can be observed in the DTG curve (Fig. 7), the partial oxidation of ferrous iron content in the crocidolite takes place in the range 200-580 °C [55], as confirmed by the three weight gain peaks at 205, 360, and 570 °C (Fig. 7). Although the detailed discussion of the processes is not the object of this paper, it is necessary to point out that the oxidation of crocidolite is essentially a dehydrogenation, as long as dehydroxylation occurs [21] so that the mechanism of oxidation was considered to be dependent on migration of protons and electrons through the crystal. Indeed, when hydrated silicates containing ferrous iron are heated the constitutional hydroxyl decomposes and iron may change its valence state. Oxidation may result from either the incorporation of oxygen into the material (oxygenation) or a dehydrogenation with the following mechanism: The DSC curve up to 700 °C showed two exothermic and two endothermic effect: 320 °C, 431 °C, 354 °C and 649 °C respectively ( Table 3). The first exothermic effect at 320 °C was related to the At 928 °C, the DSC curve shows an endothermic peak due to the conversion of magnetite to hematite. Indeed, in the DTG curve the peak at 928 °C stems from the oxidation of the ferrous iron 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 present in magnetite (Fe +2 Fe +3 O4) which involves a weight gain (Fig. 7). The DSC shoulder effect at 960 °C, which is clearly recorded on DDSC curve, was related to the incongruent melting point of the acmite with separation of hematite [57, 58]. The endothermic peak observed at 1064 °C in the DDSC curve indicated that cristobalite is being dissolved by the liquid [20]. Indeed, at 1100 °C the final minerals products detected by PXRD were mainly hematite (33-0664 JCPDS card.) while cristobalite (02-0278 JCPDS card) is evidently in a small amount because its reflections are close to the detection limit (Fig. 8).
The structural changes of crocidolite with increasing temperature can be summarized in the following steps: dehydrogenation and/or dehydroxylation accompanied by iron oxidation, structure collapse and crystallization of newly formed crystalline phases, early melting. Crocidolite, blue at room temperature, turned into dark red at 1100 °C, mostly due to hematite formation.
At SEM, the raw fibres of crocidolite appear as straight and rigid, looking like needles (Fig. 9a).
After heating at 1100 °C, the original morphology is strongly altered (Fig. 9b). The single fibres, originally arranged in fibre bundles (Fig. 9a), now appear as thick sticks, confirming that partial melting occurred during heating. However, it is still possible to recognize some fibrous-like structure that was melt-bonded ( Fig. 9b). At higher magnification (HM) crocidolite showed meltfragments composed of an aggregate of particles with totally different morphology with respect to the original morphology (Fig. 9c).
Tremolite DSC curve of tremolite asbestos from Val d'Ala (Fig. 10) exhibits a number of both endothermic and exothermic peaks in the range 500-1100 °C that can be explained by the presence of impurities in the sample [59]. The endothermic events at 729 °C and 776 °C are due to the dehydroxylation of minor chlorite and antigorite, respectively [37,36] (see Fig. 10 and Table 3). The exothermic peak at 842 °C was related to oxidation of Fe 2+ [60] present in the chlorite, its presence being also proved by the weight gain in the DTG curve. The exothermic effect at 898 °C is interpreted as 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 recrystallization to forsterite and hematite. The shoulder at 955 °C on DSC curve was ascribed to talc breakdown [38], while the sharp endothermic peak at 1046 °C corresponds to breakdown of tremolite in agreement with Luckewicz [61]. The main TG weight loss of 2.02 % between 850 and 1050 °C due to the tremolite dehydroxylation (Fig. 10, Table 2) was in agreement with the theoretical tremolite water content [62]. Finally, the exothermic DSC signal at 1077 °C was related to the crystallization of diopside. The effect on TG curve (-0.17 wt%) below 110 °C was ascribed to the release of humidity adsorbed at the sample surface. The mineral products after heating to 1100 °C were diopside (JCPDS card 11-0654), forsterite and hematite deriving from tremolite and chlorite breakdown (Fig. 8).
The TG curve of amosite showed a continuous weight loss of 1.94 % between 110 and 690 °C, due to dehydroxylation and dehydrogenation reactions [21, 26] which involve a weight gain of 0.24 % between 690 and 1000 °C, due to oxidation of ferrous iron (Fig. 11). In fact, hematite (JCPDS card 24-0072 ) was also found among the final mineral products (Fig. 12) in addition to enstatite (JCPDS card 07-0216) and quartz (JCPDS card 07-0346). The effect on TG curve (-0.33 wt%) below 110 °C was ascribed to the release of humidity adsorbed at the sample surface.
Raw amosite (Fig. 13a) shows fibres which look like flexible needle arranged in bundles. After heating at 1100 °C, the newly-formed silicate (enstatite) preserved the original fibrous morphology (pseudo-morphosis) but fibres appear more rigid and thicker (Fig. 13b). In some cases, single fibres seem to be fused together at forming prismatic crystal (Fig. 13b, 13c) and when observed at higher magnification, they appear partially covered by pseudo-spherical particles growing along the axial direction of the fibres (Fig. 13c). Figure 14 presents the thermal behaviour of anthophyllite asbestos. DTG curve shows one main peak of maximum weight loss (2.30 %) at 868 °C due to anthophyllite dehydroxylation in correspondence with the shoulder at 824 °C on DSC curve. However, DDSC confirm the presence of an endothermic peak at 861 °C which corresponds to the structural breakdown of this phase [22,27] followed by recrystallization of enstatite as showed by the exothermic peak at 915 °C (Fig. 14).
Fibres of anthophyllite asbestos appear straight, poorly flexible and thin and exhibit a slender needle-like crystal habit both before and after heating treatment (Fig. 13d, 13e). SEM images collected at higher magnification (Fig. 13f) showed that the new phase formed after heating (enstatite) preserves the original fibre morphology (pseudomorphosis) but the surface becomes rough.

Fibrous erionite
The TG curve of Fig. 15 showed a continuous weight loss due to the dehydration of erionite (H2O loss of 17.00 wt%), corresponding to the broad endothermic peak at 126 °C [29] and to the weak endothermic peak at 356 °C (Table 3) on the DSC curve. In the first endothermic effect, the water loss is 16.11 wt% while in the second endothermic the water loss is 0.89 wt%. The complete dehydration is attained at 450 °C [68] without loss of crystallinity which started at temperature above 700 °C (as verified by PXRD) followed by recrystallization of K-feldspars and plagioclase as evidenced by the DSC exothermic peak at 911 °C (Fig. 15). The curves are in agreement with those reported in Gottardi and Galli [69] and the estimated temperature of breakdown and recrystallization is comparable to that (840 °C) reported by Ballirano and Cametti [70]. Differences may be related not only to different experimental conditions but also to different Si/Al ratio, ionic potential and size of exchangeable cations [19], and crystallite size of the various samples [70]. The products of erionite recrystallization after heating to 1000 °C were K-feldspar, plagioclase and quartz (see the PXRD in Fig. 16), according to the reaction sequence Na5K3Al8Si28O7228H2O (approximated erionite formula)  Na5K3Al8Si28O72  3KAlSi3O8 + 5NaAlSi3O8 + 4SiO2.  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 At the SEM observation, erionite displays bundles composed of many fibres resembling amphibole fibres morphology (stubby prismatic and acicular crystals) (Fig. 17a). After heating treatment the new forms have feldspar-like composition as detected by EDS/SEM analyses. At low magnification SEM, imaging show that the fibres are shorter but the original fibrous morphology is preserved (Fig. 17b) and the surface is smooth. However, at higher magnification (Fig. 17c) the surface of the fibrous crystals show irregularities and appear as rather rough (Fig. 17c).
The thermogravimetric (TG) analysis allowed the calculation of the water content in the fibres which could be useful for the determination of their chemical formulae.
Despite the thermal treatment, all fibrous samples preserve the same external fibrous habit but the structure is completely changed at a molecular scale: this phenomenon called pseudomorphosis lead to the complete transformation of asbestos minerals into non-hazardous silicates such as forsterite and enstatite. However, potentially hazardous minor phases such as cristobalite and quartz were found in the new phases appeared after thermal treatment of anthophyllite asbestos, crocidolite and asbestiform erionite; these may hinder a safe reuse of the processed asbestos samples.
Moreover, it has been demonstrated that DSC and DTG analyses are very effective for the identification of minerals impurities both in chrysotile, amphibole asbestos and asbestiform erionite specimens. Indeed, DSC and DTG analysis clearly showed the presence of low impurities, not relievable by the semi-quantitative PXRD analysis, such as pyroaurite, talc, brucite, smectite, dolomite, siderite, goethite, and biotite. However, all identified minerals are non-fibrous, mostly having platy morphology.