# A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos
## 3 Results  
### SEM Characterisation  
Figure 1 shows SEM micrographs of pristine chrysotile fibres as well as fibres exposed to cryomilling for different durations. As already reported in our previous work [21], the pristine sample (CHR) is composed of very long fibre bundles (up to 1 mm in length) with split ends (Figure 1a) accompanied by clusters of intertwined fibres (Figure 1b). The fibres are very long and curly (mean L = 33.9 \(\upmu\)m) (Figure 1a). The SEM images show some important morphological changes following milling (Figure 1c-f). Although most fibres are still aggregated in bundles and clusters after 5 min of milling (CHR5), the bundle length is reduced (Figure 1c). Following 10 min of milling (CHR10), short bundles are observed together with single fibres that have apparently been separated from the bundles/clusters (Figure 1d). Increasing the milling time (10 min steps) up to a total of 40 min leads to progressively shorter fibre bundles and single fibres with decreased curliness (Figure 1e-h).  
From the analyses of SEM images, the fibre length distribution (i.e., the fraction of fibres comprised in a given length interval) was determined for pristine chrysotile and for chrysotile exposed to cryogenic milling for various durations. The results are shown in Figure 2 and Table 2. Length intervals were fixed to 0-5 \(\upmu\)m, 5-10 \(\upmu\)m, 10-15 \(\upmu\)m, 15-20 \(\upmu\)m, >20 \(\upmu\)m. As shown in Figure 2, the length of the fibres decreases as a function of milling time. Pristine chrysotile (CHR) exhibits 61.5% of the fibres longer than 20 \(\upmu\)m (Figure 2a). After 5 min of milling, this length class is reduced to a value of 49.1%, while the percentage of fibres falling within the range of 5-10 \(\upmu\)m increases from 7.5% to 21.7% (Figure 2b). After 10 min (CHR10), approximately 50% of the fibres are shorter than 10 \(\upmu\)m (Figure 2c). Increasing the milling time to 20 min (CHR20) further increases the percentage of this length class to 67% (Figure 2d). The most substantial size reduction is observed following 30 min of milling (Figure 2e). After 30 min, the fibres that belong to the range 0-5 \(\upmu\)m are 65%, while the percentage of fibres longer than 20 \(\upmu\)m is dropped to 1% (Figure 2e). Finally, after 40 min, almost all fibres (95%) are shorter than <5 \(\upmu\)m (Figure 2f).  
Since chrysotile is mainly composed of Si and Mg, the SEM-EDX semi-quantitative analyses focused on these two elements. Table 3 shows the Si and Mg content (reported in weight percentages of the oxides of the elements) of raw and ground Russian chrysotile (40 min) fibres measured by SEM-EDX. The semi-quantitative composition of the two samples is comparable. Very similar is the Si/Mg ratio, 1.05 for raw chrysotile and 1.02 for long-term milled chrysotile. Therefore, no Mg depletion was induced during cryomilling.  
Figure 1: SEM images of Russian chrysotile. (**a**) An example of the long fibre bundles in raw chrysotile (CHR). (**b**) Detail of the bundle shown in (**a**) with intertwined fibres cluster. (**c**) Chrysotile after 5 min of milling (CHR5). (**d**) Chrysotile after 10 min of milling (CHR10). (**e**) Chrysotile after 20 min milling (CHR20); (**f**) Chrysotile after 30 min of milling (CHR30). (**g,h**) Chrysotile after 40 min of milling (CHR40).  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Min** & **Max** & **Mean** & **Si/Mg** \\ \hline Raw chrysotile & & & & \\ SiO\({}_{2}\) & 47.79 & 53.61 & 50.58 (1.2) & 1.05 \\ MgO & 42.15 & 50.84 & 48.27 (2.1) & \\ \hline Chrysotile 40 min & & & & \\ SiO\({}_{2}\) & 38.74 & 55.39 & 49.46 (0.5) & 1.02 \\ MgO & 40.49 & 61.26 & 48.31 (0.7) & \\ \hline \end{tabular}
\end{table}
Table 3: Si and Mg content (expressed in terms of weight percent oxides) and Si/Mg ratio of raw and ground Russian chrysotile (40 min) fibres determined by 50 SEM-EDX spot analyses. Standard error of the mean in brackets.